A magnetic tunnel junction (MTJ) element as a magnetoresistive element has a stack structure formed with a first ferromagnetic layer, a tunnel barrier layer, and a second ferromagnetic layer. Such an MTJ element is known to have a tunneling magnetoresistive (TMR) effect, and is used in a hard disk drive (HDD) head of the 100 Mbpsi (bits per square inch) class or a magnetic random access memory (MRAM).
An MRAM is nonvolatile to store information (“1” or “0”) depending on changes in the relative angle between the magnetization directions of the ferromagnetic layers in the MTJ element. As the magnetization switching speed of the ferromagnetic layers is several nanoseconds, high-speed data writing and high-speed data reading can be performed. In view of this, MRAMs are expected to be next-generation high-speed nonvolatile memories. Further, where a technique called spin transfer torque switching is used to control magnetization with a spin polarization current, the cell size in an MRAM can be reduced, and the current density can be increased. Thus, the magnetization of a magnetic layer can be readily switching, and a high-density MRAM that consumes less power can be formed.
Recently, it has been theoretically proved that a magnetoresistance ratio as high as 1000% can be achieved where MgO is used for the tunnel barrier layer, and this has been drawing attention. Specifically, MgO is crystallized so that electrons with a certain wavenumber are selectively tunnel-conducted from a ferromagnetic layer while maintaining the wavenumber. At this point, the spin polarizability in a certain crystal orientation exhibits a large value, and accordingly, a giant magnetoresistive effect is achieved. The increase in the magnetoresistive effect of the MTJ element leads directly to an increase in the density and a decrease in the power consumption in the MRAM.
Meanwhile, to increase the density of a nonvolatile memory, a higher degree of magnetoresistive element integration is essential. However, as the device size becomes smaller, the ferromagnetic layers forming the magnetoresistive elements become poorer in thermal stability. Therefore, the problem lies in how to increase the thermal stability of ferromagnetic materials.
To counter this problem, MRAMs have recently been formed with perpendicular magnetization MTJ elements in which the magnetization directions of the ferromagnetic layers are perpendicular to the film surfaces. In a perpendicular magnetization MTJ element, a material with a high crystal magnetic anisotropy energy is normally used for the ferromagnetic layers. The magnetization of such a material is oriented in a certain crystal direction, and the crystal magnetic anisotropy energy can be controlled by changing the composition ratio between constituent elements and the crystallinities of the constituent elements. That is, the magnetization direction can be controlled by changing the crystal growth direction. Also, a ferromagnetic material has a high crystal magnetic anisotropy energy, and thus, the aspect ratio of the device can be set at 1. Further, a ferromagnetic material has a high thermal stability, and is suitable for integration. In view of these aspects, to achieve higher integration in an MRAM and reduce power consumption, it is important to manufacture perpendicular magnetization MTJ elements having a large magnetoresistive effect.